1. Field of the Invention
This invention relates to improved prosthetic joints and in particular to improved prosthetic knee joints.
2. Description of the Prior Art
Flexion and extension of the normal human knee involves complex movements of three bones: the femur, the tibia, and the patella. During flexion, the distal end of the femur and the proximal end of the tibia rotate and glide relative to one another, with the center of rotation of the joint moving posteriorly over the condyles of the femur; during extension, the tibia and femur follow the reverse path, with the center of rotation now moving anteriorly as the joint is extended. Simultaneous with these movements of the femur and tibia, the patella moves over the surface of the femoral condyles, while remaining at a relatively constant distance from the tubercle of the tibia through the attachment of the patella to the tibia by the patellar ligament.
Numerous prostheses have been proposed as replacements for the natural knee joint. See, for example, Noiles, U.S. Pat. Nos. 3,996,624, 4,219,893, and 4,301,553, Averill, U.S. Pat. Nos. 3,728,742 and 4,217,666, Insall, U.S. Pat. No. 4,213,209, Tavernetti, U.S. Pat. No. 3,813,700, German Patent Publications Nos. 2,227,090 and 2,501,128, and French Patent Publications Nos. 2,269,324 and 2,478,462. For total knee replacements, the condyles of the femur and the head of the tibia are surgically removed and replaced with prosthesis components. A patellar prosthesis, e.g., a spherically-domed or conical plastic button, is normally attached to the posterior surface of the patella to serve as an interface between the patella bone and the femoral prosthesis.
Efforts have been made to produce prosthetic joints which function in a manner similar to the natural knee. Specifically, a number of mechanisms have been proposed for producing posterior movement of the femoral component relative to the tibial component (femoral roll-back on the tibia) as the joint is flexed. For example, Walker et al., U.S. Pat. No. 4,209,861, discloses a prosthetic knee joint wherein guiding surfaces on the femoral and tibial components are used to induce posterior movement on the tibial component of the contact area between the components as the knee is progressively flexed. The posterior movement takes place through a major portion of the flexion of the joint. Burstein et al., U.S. Pat. No. 4,298,992, shows an alternate construction in which the femoral component moves posteriorly relative to the tibial component at or near full flexion. See also Deane, U.S. Pat. No. 3,840,905.
These prior art constructions suffer the common disadvantage that the femoral and tibial bearing surfaces are only in contact over small areas. Moreover, the contact areas become even smaller when the joint is flexed. During flexion, e.g., during such activities as squatting, stair climbing, or rising from a chair, high loads are applied to the joint and must be carried by the contact area between the bearing surfaces. Small contact areas plus high loads lead to high rates of wear of the bearing surfaces, which is clearly undesirable. U.S. Pat. No. 4,634,444 to Douglas G. Noiles discloses a knee joint having bearing surfaces of large areas. However, the femoral component of this joint does not move posteriorly relative to the tibial component during flexion, as occurs in the natural knee.
Efforts have also been made to improve the functioning of patellar prostheses. See, for example Pappas et al., U.S. Pat. No. 4,470,158, Buechel et al., U.S. Pat. No. 4,309,778, and Buechel et al., U.S. Pat. No. 4,340,978. In particular, the anterior surfaces of femoral components have been provided with concave recesses for receiving patellar prostheses when the joint is at or near its fully extended position. See, for example, Forte et al., U.S. Pat. No. 4,353,135, and Walker, U.S. Pat. No. 4,209,861. Similarly, the distal surfaces of femoral components have included tracks for receiving the patellar prosthesis when the joint is flexed. Significantly, the surfaces which engage the patellar prosthesis on these prior art tracks have been convexly shaped. Indeed, discontinuities in the slope of the prosthesis' outer surface have existed at the intersection between the concave recess of the anterior surface and the convex track of the distal surface.
Prostheses employing convex tracks have suffered a number of disadvantages. When used with the typical spherically-domed or conical patellar prosthesis, the track and the patellar prosthesis have only made point contact. As discussed above, prosthetic knee joints are subject to high loads when flexed, i.e., when the patellar prosthesis is in contact with the distal track. This combination of high loads and point contact has resulted in high wear rates for the patellar prosthesis. Indeed, for patellar prostheses consisting of a plastic bearing mounted on a metal backing plate, complete wear through of the bearing so as to cause the metal plate and the metal femoral component to grind against one another in situ, has been observed.
In addition to the point contact problem, the discontinuity in the outer surface of the femoral prosthesis at the intersection between the concave recess and the convex track has also contributed to wearing of the patellar prosthesis and has degraded the overall smooth operation of the prosthesis.
The Forte et al. patent referred to above discloses a construction for a patellar prosthesis which can achieve line contact with a convex track. This construction, however, employs a complex patellar button geometry which must be precisely aligned with the femoral prosthesis during the surgical procedure for the system to operate properly. Also, in revision surgery, the existing patellar prosthesis is normally not replaced. Most existing patellar prostheses are of the conical or spherically-domed button type. The Forte et al. construction, like the rest of the prior art constructions, only provides point contact when used with such spherically-domed or conical patellar prostheses.